1. Field of the Invention
The present invention relates to image processing apparatus, system and method which are used in an apparatus and a system for extracting a feature quantity from an image obtained by radiographing (e.g., X-ray photographing), and performing gradation conversion processing for the photographed image on the basis of the feature quantity. The present invention also relates to a storage medium which computer-readably stores a program of processing steps to execute the above image processing method.
2. Related Background Art
In recent years, by the advancement of a digital technology, for example, an image photographed by X-ray photographing is digitized, the obtained digital image is subjected to image processing, and the processed image is displayed on a monitor or output onto an X-ray diagnosis film.
As the above image processing, there is gradation conversion processing which converts the photographed image to have a density value easy to be observed on an output destination such as a monitor screen, the film or the like.
In the gradation conversion processing, for example, when an image photographed by X-ray photographing a lung area is output onto the X-ray diagnosis film, a histogram of all pixels constituting the photographed image is first formed, and the formed histogram is analyzed. Next, a pixel value at a point of a certain part (e.g., an upper 5% point) of the histogram is extracted as a feature quantity of the photographed image. Then conversion of a density value (i.e., gradation conversion) is performed such that the extracted pixel value (the feature quantity) becomes a certain density value (a density of 1.9 or so) on the film.
However, in such a conventional image processing method as described above, especially, the following problems are enumerated.
First, to perform the image processing to the photographed image of the lung area, conventionally the value of the point of the certain part (e.g., the upper 5% point) is extracted as the feature quantity from the histogram of the entire photographed image. This feature quantity is always extracted from the point of the certain part such as the upper 5% point or the like, irrespective of a state of a subject (e.g., a constitution of the subject) on the photographed image. Thus, even if the gradation conversion is performed based on such the feature quantity, a density distribution of the image after converting the gradation might vary according to the kind of the photographed target image. Therefore, when diagnosis is performed by using such the image which is obtained after converting the gradation and of which density distribution varies, there is some fear that a diagnosis mistake is invited. This is the serious problem.
Further, a necessary area and an unnecessary area (a transparent area or a transmission area) exist in the image photographed by the X-ray photographing. In the necessary area, the X-ray which permeated a subject hits a sensor surface, while in the unnecessary area, the X-ray directly and strongly hits the sensor surface. Incidentally, the feature quantity used for the image processing is conventionally extracted from the histogram of the entire photographed image. Namely, the feature quantity used for the image processing is extracted from information of the entire photographed image including information of the unnecessary area (the transparent area). Therefore, a desired feature quantity can not be extracted, whereby the image which is obtained after converting the gradation and is optimum for observation can not be obtained. This is also the serious problem.